In theory google ranking will not be affected by upgrading to the latest version.
Do keep in mind that there have been some changes to the pretty url system (or custom url, as it's now called)
As long as the indexed urls don't change, your search engine ranking will stay the same.
There will be at least 2 minutes of downtime. Choose a moment in the late evening, night or early morning.
Make a backup before you start upgrading, if something goes wrong, you can quickly revert to that state.
Download the latest version, unpack, upload to ftp, run the install/upgrade.php script and you should be ok.
Don't forget to update all your modules as well, right after upgrading the CMSms core.
Good luck
